1. Technical Field
The present invention relates to a sensor device, a sensor module, a force detection device, and a robot.
2. Related Art
According to a related art, a force sensor disclosed in JP-A-4-231827 is known as a force sensor using a piezoelectric material. JP-A-4-231827 discloses a force sensor in which a signal electrode is held between crystal discs of a piezoelectric material and in which plural measuring elements held between metal cover discs are arranged inside a metal ring by welding.
FIG. 10 shows a sensor device according to a related art. As shown in FIG. 10, a sensor device 200 is formed as a whole by a metallic package 202 having a sensor element 214 and a recessed section to accommodate the sensor element 214, and a metallic plate-like lid 204 which is joined to the top surface (joining surface 224) that is an outer circumference of an opening 220 of the recessed section of the package 202 and which contacts the sensor element 214.
The sensor element 214 includes a detection electrode 218 inserted between two quartz plates 216 having the same cut surface, with the quartz plates 216 facing each other.
The top surface of the quartz plate 216 is a contact surface 222 of the sensor element 214 and contacts the lid 204.
Meanwhile, a coaxial connector 206 is attached to a lateral side of the package 202. The coaxial connector 206 has an outer circumferential portion 208 and a center conductor 210. The space between these two components is filled with an insulating resin 212, and the outer circumferential portion 208 and the center conductor 210 are electrically insulated from each other. Here, the outer circumferential portion 208 is short circuited with the package 202 and the lid 204, and the center conductor 210 is electrically connected to the detection electrode 218.
This sensor device 200 is inserted between pressurizing plates (not shown) and thus pressurized. The lid 204 transmits a force (pressure) to the contact surface 222 of the sensor element 214. Then, a force (pressure) applied to the quartz plates 216 changes according to an external force applied to the pressurizing plates. The quartz plates 216 output (induce) electric charge accompanying this applied force, in the detection electrode 218 by a piezoelectric effect. Therefore, by assuming a signal output in the case of pressurization only as a reference output, and monitoring the amount of change in output electric charge due to change in the force (pressure) via the coaxial connector 206, the external force applied to the sensor device 200 can be detected.
Here, in the sensor device 200, the sensor element 214 is sealed by the lid 204 in the state where the inside of the package 202 is filled with dry air so that the electric charge induced by the quartz plates 216 does not leak because of a fall in insulation resistance due to moisture or the like.
The force sensor disclosed in JP-A-4-231827 has a structure in which a signal electrode is held between crystal discs and in which the crystal discs are held between metal cover discs. If this is welded to a metal ring, the individual parts such as the signal electrode have dimensional differences and the differences cause irregularities at the welded portion, possibly generating a gap in the weld. Therefore, if the external environment is in poor condition such as high humidity, entry of moisture into the sensor element may cause leakage of electric charge, making stable measurement difficult.
Meanwhile, in the related-art sensor device shown in FIG. 10, the height of the contact surface 222 of the sensor element 214 accommodated in the package 202 and the height of the joining surface 224 that is the outer circumference of the opening 220 of the recessed section of the package 202 may not coincide with each other in some cases.
FIGS. 11A to 11D show schematic views of the related-art sensor device (the height of the contact surface>the height of the joining surface). FIGS. 11A and 11B are plan and sectional views before the lid is joined. FIGS. 11C and 11D are plan and sectional views after the lid is joined. Here, for simplification, the quartz plates and detection electrode as components of the sensor element 214, and the wire and connector for taking out a signal outside the package are not shown.
As shown in FIGS. 11A and 11B, before the lid 204 is joined to the package 202, the sensor element 214 is aligned with and placed at the center of the recessed section of the package 202, and the lid 204 is placed thereon to cover the opening 220 of the recessed section of the package 202. At this point, since there are manufacturing variances in the heights of the package 202 and the sensor element 214, a gap 600 may be formed between the lid 204 and the joining surface 224.
Next, as shown in FIG. 12, roller electrodes 280 are pressed against positions (joining surface 224) on the lid 204 that connect to the package 202, and a current is applied to the roller electrodes 280 to join and airtightly seal the lid 204 and the package 202 by seam welding.
When this lid 204 is seam-welded, the sensor element 214 and the lid 204 may be misaligned by vibrations in a carrying process or the pressing force of the roller electrodes 280, and the lid 204 is often joined to the package 202 with a misalignment as viewed in a plan view, as shown in FIG. 11C. In such a case, on the joining surface 224 between the lid 204 and the package 202, a portion (insufficient joining area 270) where the lid 204 and the package 202 are joined together with only a very short length left between the inside and outside of the package is formed, as shown in FIG. 11D. This insufficient joining area 270 has low stress resistance. Therefore, in the sensor device 200, to which a force is applied repeatedly, repetition fatigue may break the joining between the lid 204 and the package 202 and break the airtight sealing, thus impairing reliability.
Meanwhile, if the contact surface between the package 202 and the sensor element 214 and the contact surface between the sensor element 214 and the lid 204 are bonded with an adhesive in order to prevent misalignment of the sensor element 214 and the lid 204 at the time of seal welding, a creep phenomenon due to pressurization causes a long-term reduction in the thickness of the adhesive layer, which causes a fall in applied pressure. Thus, there is a problem that force detection characteristics may change or force detection may become impossible. Thus, in view of the characteristics and reliability of the sensor device, it is not preferable to use an adhesive to bond the contact surface between the package 202 and the sensor element 214 and the contact surface between the sensor element 214 and the lid 204.